This invention relates to damping undershoot in current write drivers in magnetic disk drives.
Data are recorded in magnetic media of magnetic disk drives by reversing current through a magnetic head to reverse magnetic flux generated by the head. The flux reversals are recorded as a "transition" in the recording medium adjacent the head, each transition representing binary data. As technology advances to increase the density of data recorded on the medium, the need arises for shorter current pulses, requiring quicker current rise and settling times at the current reversals.
Current reversal in a write head is effectuated by a current driver, operable to drive the current in opposite directions through the head. One common such driver is an H-switch, configured of four transistor switches operable in pairs such that two transistors conduct to direct write current in one direction through the head, and the other two transistors conduct to direct write current in the opposite direction through the head. The reversal of current through the head reversed flux generated by the head to record data in the magnetic medium.
The transistors of the H-switch exhibit parasitic capacitance that stores a charge during the steady-state condition of the H-switch. However, at the time of current reversal (current transition) by the H-switch, the parasitic capacitances charge or discharge, affecting the current waveform during transition. More particularly, discharge of the parasitic capacitance into the write current generates a current overshoot through the head. The overshoot generates a subsequent current undershoot condition, which generates ringing of the write current waveform until the write current ultimately settles to the steady-state condition.
While overshoot is not necessarily an adverse condition, as overshoot often leads to quicker rise times to the current transition, overshoot causes undershoot conditions which generates ringing and deteriorates the current waveform. The present invention is directed to an active damping circuit for damping current undershoot, thereby minimizing ringing.